Coulomb‐induced intramolecular exciplex formation in semiflexible donor‐bridge‐acceptor compounds in nonpolar solvents as a function of temperature

Abstract

Abstract The mechanism of exciplex formation of two donor‐bridge–acceptor compounds, both containing a strong donor ‐ acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohexane show that in both compounds the exciplex is formed via the harpooning mechanism, i.e. long‐range charge separation leading to an extended charge‐transfer state, followed by Coulomb‐induced folding to the exciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furthermore, the activation energy of the folding process was estimated from the decay rates of the extended charge‐transfer state in various solvents. An interpretation of the observed activation energies in terms of Kramers theory is presented, which gives an indication of the relative importance of internal steric effects and solvent viscosity on the dynamics of the folding process.